A deterministic model of cyclical selection in randomly mating populations is studied. Sufficient conditions for a protected polymorphism, which are for the special case of alternating selection also necessary conditions, are obtained using a simple graphical approach. The most important condition requires ‘marginal overdominance’ (Wallace, 1968); the other conditions seem hard to satisfy in a natural situation. Furthermore it is shown that the cyclical selection model can be regarded as a special case of a frequency-dependent selection model (Cockerham et al. 1972). Using this property, a mean fitness function for the cyclical selection model is derived. Generally, the mean fitness will not be maximized under cyclical selection. The relevance of the model to the problem of the role of cyclical selection in the maintenance of genetic polymorphism in natural populations is discussed. It is concluded that this relevance is probably rather limited with regard to the creation of protected polymorphism, but that the influence of cyclical selection on transient polymorphisms might be more significant. An approximate formula for the time needed for a given change in gene frequency under cyclical selection is derived. It appears that cyclical selection can extend considerably the time during which a transient polymorphism persists, especially if the selective differences in the different environments are of the same order of magnitude and of opposite sign.

An experiment was carried out to test the effect of varying selection intensity on the response to individual selection with a fixed number of individuals scored per generation. The selected trait was egg laying of virgin females of Tribolium castaneum scored from the 7th to the 1 lth day after adult emergence. Five different selected proportions of females were considered (5, 10, 20, 33 and 50%) and each treatment was represented by two replicate lines. Control lines were maintained throughout the experiment. The lines selected at the lowest proportions (5 and 10%) led to the largest initial gains, but the largest final gains were achieved, by the lines where the proportions selected were 10 and 20%, in agreement with theory. Lines where the proportion selected was 50% gave the lowest rate of response over the period considered (32 generations). A good agreement was found between predicted and realized short-term responses to selection. Prediction at later stages of selection deteriorated in the most strongly selected lines mainly due to the levels of inbreeding attained.

Twelve FPA-resistant mutants were selected on medium containing p-fluorophenylalanine and ethionine. Dominance tests in heterozygous diploids showed that 8 out of 12 are dominant and 4 recessive to their wild-type alleles. One mutant, fpa60, showed a partial requirement for tyrosine and was found to be allelic to an fpaA mutant described previously. A tyrosine non-requirer, fpa65, was also assigned to this locus. The other 10 mutants did not show any growth requirement and were simultaneously resistant to ethionine and 3-amino-L-tyrosine. Of the 8 dominant mutants, 3 were allelic to the permease-mutants at the locus fpaD. Dominant mutants showed higher degrees of resistance than recessive ones. Six new loci, identified after preliminary genetic analysis, were located on 3 linkage groups: 3 on linkage group VI, and one each on linkage groups I, V, and VIII. The recombinant fpaD11; fpaK69 was found to be sensitive to FPA.

To test whether exrB, a mutation in Escherichia coli B making this strain sensitive to ultraviolet radiation and unable to divide normally, is dominant to exrB+, merodiploids were constructed by crossing Hfr exrB metA+his × F−malB met A thr proA recA. metA+recombinants were fertile (F′) and sensitive to UV. One of these, F′ exrB proA, was crossed with an F−metA malB thr, selecting for metA+proA+. All such progeny acquiring malB+ were UV-sensitive, fertile and segregated met mal UV-resistant progeny. They were, therefore, meriodiploids, F′ exrB/exrB+. exrB is dominant to exrB+, in which respect it resembles exrA (lex).

Nine (41%) of the 22 enzymic and non-enzymic loci examined in a strain of mice divergently selected for six-week body weight (six lines selected in each direction and six controls) were found to be polymorphic. The degree of polymorphism varied between the replicates from a maximum of 38% to a minimum of 14% with an average individual heterozygous at 7·7% of its loci. There was no obvious association between any of the isozyme variants and body size. The frequency distribution among the 18 lines was adequately accounted for by random genetic drift. However, an association was observed between body size and the Hbb locus; the Hbbs allele was found to be fixed in all of the six Large lines. An examination of the variance of gene frequencies at this locus excluded random genetic drift as an explanation for the fixation.

Ethidium bromide and acriflavin-resistant mutants of petite-negative yeast Kluyveromyces lactis were prepared. One kind of nuclear mutation (EBR1) gave resistance to ethidium bromide and correlated sensitivity towards acriflavin. Another nuclear mutation (EBR2) did not affect ‘natural’ resistance of this yeast towards 15 μM acriflavin. Both nuclear mutations mapped at different loci, suggesting lack of linkage. Cytoplasmic mutants resistant to these two drugs were unstable when grown in complete media with dextrose, reverting to a wild-type resistance genotype. When grown in glycerol-containing media these mutants maintained their cytoplasmic drug-resistance conferring factors.

Interrupted mating methods were used to map the positions on the chromosome of genes for the utilization of acetamide, histidine and proline. An entry time of 22 min was found for a put marker determining the utilization of proline. Two groups of hut mutants were studied. Mutation hut-109 was located at 18 min and mutation hutU108, which results in a defect in urocanase activity, mapped at 28 min. hutU was shown by transduction with bacteriophage F116 to be linked to ser-3 but not to met-28 and ilv-202, giving a map order of hutU-ser-3-ilv-202-met-28.

The amidase genes were assigned an entry time of 65–75 min. A method is described for examining late entry markers by interrupted mating.

Two strains of poultry selected for high growth-rate were found to have an anomalous lens morphology indicating a failure of the normal process of growth regulation. The implications for lens fibre differentiation are discussed, as are the implications for genetic selection for growth-rate.

Mouse chimaeras made by aggregating embryos homozygous for nine recessive genes with embryos carrying the corresponding dominants were mated with partners of the recessive strain. Progeny were either of the recessive type for all loci examined, or showed the dominant characters at all loci. Tail length in homozygous vestigial-tail progeny was unaffected by whether the fertilizing spermatozoon had undergone maturation in a chimaera or in a control vestigial-tail male. There was thus no indication that the germ cells had been in any way modified by their intimate association with germ cells and somatic tissue of contrasting genotype in the chimaeras.